1. Field of the Invention
The present invention relates to methods and apparatus of reducing and/or controlling vortex-induced-vibrations (“VIV”) due primarily to ocean and other currents, low-frequency drift oscillations due to random waves, and low frequency wind-induced resonant oscillations of moored offshore platforms and other marine elements. In another aspect, the present invention relates to methods and apparatus for the use of thrusters for the control of VIV, low-frequency drift oscillations due to random waves, and low-frequency wind induced resonant oscillations. In even another aspect, the present invention relates to methods and apparatus for the active control of VIV, low frequency drift oscillations due to random waves, and low-frequency wind induced resonant oscillations. In still another aspect, the present invention relates to use of thrusters in combination with feedback control for the active control of VIV, low-frequency drift oscillations due to random waves, and low-frequency wind-induced resonant oscillations.
2. Description of the Related Art
The development of oil and gas reserves in deep water (over 1300 feet deep) and ultra-deep water (over 2000 feet deep) has required the design and construction of floating drilling and production platforms or vessels that do not rest on the ocean bottom. These floating platforms include but are not limited to tension leg platforms, spars, semi-submersibles, and Floating Production, Storage and Offloading (FPSO) vessels.
The floating platforms or vessels are moored to the sea floor, usually with conventional catenary mooring lines. One of the problems faced by these vessels is that they must be maintained in a relatively small circle of movement above the sea floor, called the watch circle, to avoid breaking drilling equipment or production risers extending from the vessel to the sea floor.
Hampering maintenance of a small watch circle for the floating platforms and vessels is the effects of ocean currents and random waves and wind on those structures. Ocean currents flowing past the structures can cause vortex-induced vibrations of those structures. Risers, drilling equipment and mooring lines may also develop VIV. In addition, random waves and wind striking the structures can cause low-frequency oscillations of the structures, which can move the structures relative to the sea floor and stress connections with the sea floor.
Methods and equipment developed to reduce vibrations and oscillations of these floating structures include attaching strakes or shrouds to the structures to reduce VIV. Methods and equipment developed to reduce the rolling and transverse swaying of FPSO vessels include modified “bilge keels”, which are perpendicular plates extending from the bottom, or bilge, of the ship.
In spite of advancements in the art, there is a need in the art for methods and apparatus for controlling VIV and low frequency oscillations of marine elements.
There is another need in the art for methods and apparatus for controlling VIV and low frequency oscillations of marine elements that do not suffer from the disadvantages of the prior art.
There is even another need in the art for methods and apparatus for controlling VIV and low frequency oscillations of marine elements that can be activated only when the vibration or other oscillation is occurring.
There is still another need in the art for methods and apparatus for controlling VIV and low frequency oscillations of marine elements that can be modulated for the extent of the vibration or other oscillation.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawing and claims.